1. Field of the Invention
This invention relates to a receiver for information transmitted on a telephone line. More particularly, it relates to the reception of call related information, e.g., Caller ID information, on a line side using an audio codec.
2. Background of Related Art
The reception of call related information before answering an incoming call is known. For instance, one call related information service is called Calling Identity Delivery (Caller ID), which is a well known and popular service provided by many telephone companies. This service provides the telephone number and household name information of a calling party to the called party before the call is answered. Based on a display of the Caller ID information, the called party may decide not to answer the incoming call. Basic Caller ID information is transmitted from the local telephone company to the called party while the called party's phone is in a hung-up or on-hook state, between the first and second rings. Customer premises equipment capable of receiving Caller ID (CID) information including Calling Number Delivery (CND) and Calling Name Delivery (CNAM) when the customer premises equipment is on-hook is generally referred to as Type 1 customer premises equipment.
Waveform (a) of FIG. 6 depicts the signal on a telephone line including Caller ID information transmitted from a central office to customer premises equipment. As depicted, the Caller ID information is transmitted to the customer premises equipment as marks and spaces between a first and second ring signal. Waveform (b) of FIG. 6 shows the approximate current draw allowed by customer premises equipment from a telephone line while the customer premises equipment is on-hook or hung-up, according to current standards in the United States. Although the customer premises equipment is permitted to draw additional power from the telephone line approximately during and between the first and second ring signals while the customer premises equipment is on-hook, even this additional power is extremely limited both in amplitude and in duration. Moreover, although customer premises equipment is permitted to draw significantly more power while in an off-hook condition, Type 1 Caller ID information is received entirely while the customer premises equipment is on-hook. Because of the limited amount of current available from the telephone line while the customer premises equipment is in an on-hook state, most devices which receive Caller ID information are either powered by an external power source, or are generally non-linear analog circuits.
Conventional circuits exist for receiving call related information on a line side of a telephone circuit. For instance, FIG. 7 shows a conventional analog circuit for receiving call related information, e.g., Caller ID.
In FIG. 7, call related information is coupled through capacitors 702, 704 and transformer T1 even when the on-hook switch 706 is open. Ring signals are detected by opto-isolator OPTO1. The circuit of FIG. 7 is entirely analog. Isolation between the high voltage, line side of the circuit (the right side in FIG. 7) and the low voltage side of the circuit (the left side in FIG. 7) is provided by transformer T1.
FIG. 8 shows another conventional circuit for receiving call related information. In FIG. 8, the circuit for receiving the call related information, e.g., Caller ID, is an analog circuit separated from the main signal path of the telephone circuit. The call related information signal path is isolated by isolation capacitors 808, 810, and differentially input to an operational amplifier (op amp) 820 via resistors 804, 806. The output of the op amp 820 is input to a codec 830, which digitizes the signal for processing by the digital signal processor (DSP) 840.
In FIG. 8 the call related information circuit is an analog circuit with the codec 830 being on the low voltage side of isolation capacitors 808, 810. The codec 830 is powered by an external source in the conventional circuit of FIG. 8. Moreover, ring signals may be detected either by the opto-isolator 708 or through the call related information signal path.
Call related information receiving circuits such as those shown in FIGS. 7 and 8 require extra circuits or require that an audio codec, e.g., 730 (FIG. 7) and 830 (FIG. 8), be in a powered-on condition which is wasteful. For example, a call related information device may be battery or line powered, relocated easily from place to place, and installed in any convenient telephone jack. For convenience purposes it may be desirable to not provide the call related information device with an external power source. Thus, in these type situations, extra circuits or audio codecs may be wasteful of available battery or line power.
Similarly, FIG. 9 shows yet another conventional circuit for receiving call related information that requires extra circuits. In FIG. 9, the circuit for receiving call related information is a digital circuit separated from the main signal path of the telephone circuit.
In particular, as shown in FIG. 9, a tip signal T and a ring signal R from a telephone line are input to a filter 918 and an analog amplifier 916, which amplifies the analog signal including the call related information from the telephone line. The output of the analog amplifier 916 is input to a limit/slice circuit 914, which forms a digital signal for processing by the OOK modulator 912. The OOK modulator 912 provides on-off keying at the FSK rate to pass the call related information to the DSP 940 through isolation capacitors 902, 904. The DSP 940 includes an FSK decode module to decode the call related information and output the call related information, e.g., Caller ID data including CND and CNAM, for storage in log memory and/or for display.
In FIG. 9, the codec 930 remains on the low voltage side of the telephone circuit, i.e., isolated from the telephone line, in this case by isolation capacitors 902, 904 and transformer T1. The codec 930 is powered by an external source, and is not in the path of reception of call related information. Ring signals may be detected either by the opto-isolator 708 or through the call related information signal path, but in either case amplitude information with respect to the ring signal is not detected.
There is a need for low power elements allowing processing in a digital call related information signal path on a high voltage side of a telephone circuit.